1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems provide wireless connectivity using base stations or other wireless entities such as access points, base station routers, and the like. For example, a mobile unit may establish a wireless communication link over an air interface with a base station that is a communicatively coupled to a network. The mobile unit may use the wireless communication link to access services provided by the network such as establishing a communication session with another mobile unit. The information transmitted using the communication session between the two mobile units may be analog or digital information and the communication path between the mobile units may be formed using a circuit-switched architecture or a packet-switched architecture. In a circuit-switched architecture, a dedicated communication path is formed between the two mobile units and may only be used by the two mobile units. In contrast, packet-switched architectures divide the information up into packets that can be transmitted along numerous paths between the two mobile units using a common packet network infrastructure for forwarding the packets between the mobile units and their network peers. Thus, some or all of the paths through a packet-switched network infrastructure may be shared by other mobile units or other entities coupled to the packet-switched network such as a network server or a fixed subscriber.
Voice over Internet Protocol (VoIP) is a technique for encoding audio signals (such as voice signals) into a digital format that can be used to form packets for transmission over a packet-switched network. The VoIP packets are typically referred to as delay-intolerant information because large delays between successive packets at the destination VoIP session peer (e.g., a mobile unit) may degrade the quality of the audio signal produced by the source peer. Consequently, VoIP applications are typically constrained to provide VoIP packets at a selected quality-of-service (QoS) level. For example, a VoIP application implemented in a mobile unit may be required to maintain minimum levels of delay, latency, and the like for packets transmitted over the network. In some cases, customers may pay larger fees to obtain overall higher QoS levels of higher QoS levels for certain applications.
Numerous wireless access technologies may be used to support packet data applications. Some exemplary wireless access technologies include second generation (2G), third generation (3G), and fourth generation (4G) technologies such as IX-EVDO, UMTS and WIMAX. These wireless access technologies operate according to standards and/or protocols such as the standards and/or protocols established by the Third Generation Partnership Project (3GPP, 3GPP2) and WiMAX Forum Network Working Group (NWG). Equipment vendors are developing and deploying dual mode (or multi-mode) mobile units that are capable of communicating using multiple wireless access technologies. For example, a dual-mode mobile unit may implement two independent means of IP connectivity that operate according to two different wireless access technologies. At the same time, service providers are increasingly using more than one wireless access technology to provide wireless connectivity. For example, some service providers have deployed heterogeneous networks that include overlaid meshes that operate according to different access technologies. The overlaid meshes may be used as part of an evolution from a legacy technology to a newer technology or for other reasons, such as reducing deployment and/or operating costs, improving the overall communication spectrum characteristics, and the like.
Individual mobile units may frequently handoff between base stations that operate according to different wireless access technologies as the mobile unit moves through a heterogeneous network. For example, a mobile unit may initially be using a wireless communication link formed according to the EV-DO standards and/or protocols for a VoIP call. The mobile unit may then determine that the signal quality of the EV-DO wireless communication link has degraded and may elect to handoff to a base station that operates according to the WIMAX communication standards and/or protocols. Conventional handoff protocols attempt to maintain seamless connectivity of the VoIP session as it is handed off from the fading EV-DO wireless communication link to the stronger WIMAX link. However, conventional handoff protocols do not maintain the QoS level of the call during the handoff.